Membrane technologies such as microfiltration (MF), ultrafiltration (UF), nanofilitration (NF), and reverse osmosis (RO) have been widely used for water purification because they are energy efficient, cost-effective and simple to operate. However, many commercial membranes experience substantial flux declines when they are exposed to contaminants such as, for example, inorganic salts, emulsified oil droplets, and natural organic matter. These contaminants in the water can deposit on the membrane surface and/or block the pores of the membrane, which foul the membrane and decrease its useful life.
Current approaches to address this fouling problem include pretreatment of the feed water, periodic depressurization of the membrane tube, flow reversal, and use of cleaning agents to remove fouled films from membrane surface. These techniques require energy and/or additional chemicals, and reduce productive membrane operating time, which directly contributes to increased operating costs.
Modifying a surface of the membrane with a fouling-resistant material is another approach that can potentially increase the useful life of the membrane and reduce operating costs. Materials such as, for example, nanoparticles, enzymes, and epoxy compounds, have demonstrated some fouling resistance, but there is still a need to develop highly water-permeable, anti-fouling materials that retain high water flux through the membrane over an extended membrane operating time.
Quarternary ammonium salts have anti-microbial properties, and poly(ethylene glycol) (PEG) has been used to reduce organic or bio-fouling. UF and MF membranes with surface-grafted copolymer brushes of mono-functionalized PEG and cross-linkable quarternary ammonium compounds exhibited some antibacterial and anti-fouling properties. However, attaching the brushes to the membrane required multiple reaction steps to initiate polymerization from the membrane surface, which limits the usefulness of the surface grafting technique to a narrow range of specific membranes.